I would ideally like to make 7 knockdown targets electronic, so when they are knocked down they will automatically come back up OR have a option where they stay down when hit until you want to bring them back up.

How hard would this be? Ideally I wouldn't want to spend much. I already have 7 knockdown targets if that makes a difference
Quite easy, perhaps the servo controller that Mark used would be the easiest option but I'm not familiar with the programming language used, looking at the code I wouldnt imagine its that hard.
Using the controller I use would be quite easy (but isnt anything if you know how!) its just a matter of something to detect the drop of the target, then when that happens something to pop it back up again. The system I use uses 2 servos for the target to go up and down, the reason I used 2 was to make the mechanics easier.
One servo simple pulls a magnet on a spring to one side and thus releases the target and allows it to "stand up" the second one pulls it down.
Cost wouldnt be immense I wouldnt have thought - servos that do the job for about £10 or so each (depending on the weight of the target and how much torque is needed to lift / drop it) Heaver target = more torque needed = more £££.
Sensors to detect the target dropping, perhaps a simple microswitch would do if it was dome in such a way as to not directly whack the switch. A controller board and a few bits and bobs on top probably brings the total to under £100 at a rough guess.
Making them pop up when you want can be easier or more difficult, switches connected to the target/controller via a cable would be cheap but messy, but remotely it starts getting more difficult and more expensive.

My thoughts of course, maybe someone has an easier solution.